1. Field of the Invention
The present invention relates to a non-volatile semiconductor switch circuit which can store an ON/OFF state after a power supply has been disconnected, and reproduce that original state when the power supply is reconnected.
2. Description of the Related Art
The invention described in the following Reference serves as an example of conventional technology in the field of the present invention.
Reference: Japanese Patent Application Laid-Open (JP-A) No. 3-150796
FIG. 2 is a circuit diagram of a conventional non-volatile semiconductor switch circuit, as described in the Reference.
In this non-volatile semiconductor switch circuit, a P-channel MOS transistor 3 (hereinafter called a xe2x80x9cPMOSxe2x80x9d) and an N-channel MOS transistor 4 (hereinafter called an xe2x80x9cNMOSxe2x80x9d) are connected between input/output terminals 1 and 2. The PMOS 3 and the NMOS 4 control an ON/OFF connection condition between the input/output terminals 1 and 2. Gates of the PMOS 3 and the NMOS 4 are connected to control terminals 5 and 6 respectively.
A flip-flop formed by inverters 7 and 8 is connected at the control terminals 5 and 6. That is, an input of the inverter 7 is connected to the control terminal 5 and an output of the inverter 7 is connected to the control terminal 6, while an input of the inverter 8 is connected to the control terminal 6 and an output of the inverter 8 is connected to the control terminal 5.
Respective one terminals of ferroelectric capacitors 9 and 10 are connected to the control terminals 5 and 6. Respective other terminals of the ferroelectric capacitors 9 and 10 are connected to a control terminal 11. The ferroelectric capacitors 9 and 10 are also connected to a substrate potential VSS via capacitors 12 and 13, respectively.
FIG. 3 shows a hysteresis characteristic of a ferroelectric capacitor. The horizontal axis represents applied voltage and the vertical axis represents a polarization within the ferroelectric capacitor that results from application of voltage. A hysteresis curve is shown by solid line A in FIG. 3. For example, if a positive voltage exceeding a certain value is applied and then the applied voltage returns to 0 V, a positive polarization persists at the ferroelectric capacitor. Further, if a negative voltage exceeding a certain value is applied and then the applied voltage returns to 0 V, a negative polarization persists at the ferroelectric capacitor. Hence, the state of the voltage most recently applied to the ferroelectric capacitor can be preserved by the persisting polarization state.
Operation of the non-volatile semiconductor switch circuit of FIG. 2 is performed as described below.
A control pulse at the substrate potential VSS is applied to the control terminal 5 and a control pulse at a power supply voltage VCC is applied to the control terminal 6. Hence, the PMOS 3 and the NMOS 4 both turn on, and there is an ON condition between the input/output terminals 1 and 2. Also, the levels of the control pulses applied to the control terminals 5 and 6 are retained at the inverters 7 and 8. Even after the control pulses are removed, the ON condition between the input/output terminals 1 and 2 is maintained. Then, when a driving pulse at a level of half of the power supply voltage VCC is applied to the control terminal 11, a voltage of +VCC/2 is applied across the terminals of the ferroelectric capacitor 9 and a voltage of xe2x88x92VCC/2 is applied across the terminals of the ferroelectric capacitor 10. When the driving pulse is subsequently removed, polarizations persist at the ferroelectric condensers 9 and 10 in accordance with the voltage of the applied driving pulse.
Thus, when the power supply for the non-volatile semiconductor switch circuit is disconnected, the control pulse condition retained by the inverters 7 and 8 is lost. However, even after disconnection of the power supply, the polarization states of the ferroelectric capacitors 9 and 10 are preserved.
Then, when the power supply for the non-volatile semiconductor switch circuit is to be reconnected, a voltage of VCC/2 is applied to the control terminal 11 before the power supply is turned on. Thus, voltages corresponding to the polarization states preserved at the ferroelectric capacitors 9 and 10 are output at the one terminals of the ferroelectric capacitors 9 and 10, and the control terminals 5 and 6 are brought to potential levels substantially the same as the most recent levels before the power supply was disconnected. In this condition, the power supply is connected, the inverters 7 and 8 are set by a potential difference between the control terminals 5 and 6, and the condition of the circuit before the power supply was disconnected is reproduced.
Because the ferroelectric capacitors preserve polarizations caused by applied voltages, they can store information. When information is rewritten, it is necessary to reverse the polarizations. As the solid line A in FIG. 3 shows, ferroelectric capacitor hysteresis curves usually have point symmetry with respect to a 0 V applied voltage.
However, if a voltage is applied to a ferroelectric capacitor for a long time, an effect known as xe2x80x9cimprintingxe2x80x9d occurs, by which the hysteresis curve is displaced along the applied voltage axis, as shown, for example, by dotted line B in FIG. 3. When the hysteresis characteristic has been changed in the manner of the dotted line B by imprinting, preserving positive polarization is more difficult than it was with the original hysteresis characteristic of the solid line A, and a larger voltage must be applied in order to reverse a negative polarization. When there is charge at both electrodes of a ferroelectric capacitor, electric fields are generated in the ferroelectric material, and imprinting tends to occur.
When the power supply to the non-volatile semiconductor switch circuit of FIG. 2 is disconnected, because the terminals of the ferroelectric capacitors 9 and 10 are not directly connected to each other, it is not possible to set the voltage thereacross to 0 V. Consequently, there are electric fields in the ferroelectric capacitors 9 and 10 and, if the power supply is continuously disconnected for a long time, imprinting is likely to occur and it will be difficult to maintain the stored information.
The present invention is provided to solve the above-described problems of conventional technology, and an object of the present invention is to provide a non-volatile semiconductor switch circuit that can assuredly preserve a condition from before disconnection of a power supply even when the power supply is continuously disconnected for a long time.
A first aspect of the present invention is a non-volatile semiconductor switch circuit having: a signal preservation section for preserving complementary control signals generated in accordance with a first potential and a second potential which are applied at a first control terminal and a second control terminal respectively; a switch section connected to the signal preservation section and formed by complementary transistors between a first input/output terminal and a second input/output terminal for switching between an on state and an off state in response to the control signals; a first ferroelectric capacitor connected between the first control terminal and a third control terminal for generating a polarization state in accordance with a potential difference between the first and third control terminals when a third potential is applied at the third control terminal and for preserving the polarization state after the first and third potentials have been removed; a second ferroelectric capacitor connected between the second and third control terminals for generating a polarization state in accordance with a potential difference between the second and third control terminals when the third potential is applied at the third control terminal and for preserving the polarization state after the second and third potentials have been removed; a first short section connected in parallel with the first ferroelectric capacitor for placing input/output terminals of the first ferroelectric capacitor in a short-circuited state when a preservation signal is applied; and a second short section connected in parallel with the second ferroelectric capacitor for placing input/output terminals of the second ferroelectric capacitor in a short-circuited state when the preservation signal is applied.
When the third potential is applied to the third control terminal and the preservation signal is applied to the first and second short sections such that the input/output terminals of the first and second ferroelectric capacitors are in short-circuited states, programming of the switch section is performed by applying the first and second potentials at the first and second control terminals, respectively, to thereby operate the signal preservation section and by temporarily stopping the preservation signal to thereby temporarily release short-circuited states of the first and second ferroelectric capacitors.
Further, when the third potential is applied to the third control terminal and the preservation signal is stopped such that short-circuited states of the first and second ferroelectric capacitors are released, a state of the switch section recorded by programming is read out by applying a potential, which is higher than the third potential, at the third control terminal to thereby operate the signal preservation section.
The first and second short sections are each formed by one of an enhancement-type field effect transistor and a depletion-type field effect transistor.
A second aspect of the present invention is a non-volatile semiconductor switch circuit having: a signal preservation section for preserving complementary control signals generated in accordance with a first potential and a second potential which are applied at a first control terminal and a second control terminal respectively; a switch section connected to the signal preservation section and formed by complementary transistors between a first input/output terminal and a second input/output terminal for switching between an on state and an off state in response to the control signals; a first ferroelectric capacitor connected between the first control terminal and a third control terminal for generating a polarization state in accordance with a potential difference between the first and third control terminals when a third potential is applied at the third control terminal and for preserving the polarization state after the first and third potentials have been removed; a second ferroelectric capacitor connected between the second and third control terminals for generating a polarization state in accordance with a potential difference between the second and third control terminals when the third potential is applied at the third control terminal and for preserving the polarization state after the second and third potentials have been removed; a first short section connected in parallel with the first ferroelectric capacitor for placing input/output terminals of the first ferroelectric capacitor in a short-circuited state when a preservation signal is applied; a second short section connected in parallel with the second ferroelectric capacitor for placing input/output terminals of the second ferroelectric capacitor in a short-circuited state when the preservation signal is applied; a first switching element connected between the first and third control terminals for switching between an on state and an off state; a second switching element connected between the second and third control terminals for switching between an on state and an off state; a third switching element connected between the first control terminal and the first ferroelectric capacitor for switching between an on state and an off state in accordance with the preservation signal; and a fourth switching element connected between the second control terminal and the second ferroelectric capacitor for switching between an on state and an off state in accordance with the preservation signal.
When the first through fourth switching elements are in off states and the third potential is applied to the third control terminal and the preservation signal is applied to the first and second short sections such that the input/output terminals of the first and second ferroelectric capacitors are in short-circuited states, programming of the switch section is performed by applying the first and second potentials at the first and second control terminals, respectively, to thereby operate the signal preservation section and by temporarily stopping the preservation signal to thereby temporarily release short-circuited states of the first and second ferroelectric capacitors and temporarily place the third and fourth switching elements into on states.
Further, when the first through fourth switching elements are in off states and the third potential is applied to the third control terminal and the preservation signal is applied to the first and second short sections such that the input/output terminals of the first and second ferroelectric capacitors are in short-circuited states, a state of the switch section recorded by programming is read out by temporarily placing the first and second switching elements into on states, and then stopping the preservation signal to thereby release short-circuited states of the first and second ferroelectric capacitors and place the third and fourth switching elements into on states and applying a potential, which is higher than the third potential, at the third control terminal to thereby operate the signal preservation section.
The first and second short sections are each formed by one of an enhancement-type field effect transistor and a depletion-type field effect transistor. Further, the first through fourth switching elements are each formed by one of an enhancement-type field effect transistor and a depletion-type field effect transistor. The circuit may also include: a first capacitor connected at one side thereof to the first control terminal and the third switching element and connected at another side thereof to earth; and a second capacitor connected at one side thereof to the second control terminal and the fourth switching element and connected at another side thereof to earth.
A third aspect of the present invention is a non-volatile semiconductor switch circuit having: a signal preservation section for preserving complementary control signals generated in accordance with a first potential and a second potential which are applied at a first control terminal and a second control terminal respectively; a switch section connected to the signal preservation section and formed by complementary transistors between a first input/output terminal and a second input/output terminal for switching between an on state and an off state in response to the control signals; a first ferroelectric capacitor connected between the first control terminal and a third control terminal for generating a polarization state in accordance with a potential difference between the first and third control terminals when a third potential is applied at the third control terminal and for preserving the polarization state after the first and third potentials have been removed; a second ferroelectric capacitor connected between the second and third control terminals for generating a polarization state in accordance with a potential difference between the second and third control terminals when the third potential is applied at the third control terminal and for preserving the polarization state after the second and third potentials have been removed; a first short section connected in parallel with the first ferroelectric capacitor for placing input/output terminals of the first ferroelectric capacitor in a short-circuited state when a preservation signal is applied; a second short section connected in parallel with the second ferroelectric capacitor for placing input/output terminals of the second ferroelectric capacitor in a short-circuited state when the preservation signal is applied; a first switching element connected between the first and third control terminals for switching between an on state and an off state; a second switching element connected between the second and third control terminals for switching between an on state and an off state; a third switching element connected between the first control terminal and the first ferroelectric capacitor for switching between an on state and an off state in accordance with a signal other than the preservation signal; and a fourth switching element connected between the second control terminal and the second ferroelectric capacitor for switching between an on state and an off state in accordance with a signal other than the preservation signal.
When the first through fourth switching elements are in off states and the third potential is applied to the third control terminal and the preservation signal is stopped such that the input/output terminals of the first and second ferroelectric capacitors are in short-circuited states, programming of the switch section is performed by applying the first and second potentials at the first and second control terminals, respectively, to thereby operate the signal preservation section and by temporarily placing the third and fourth switching elements into on states.
Further, when the first through fourth switching elements are in off states and the preservation signal is applied to the first and second short sections such that the input/output terminals of the first and second ferroelectric capacitors are in short-circuited states, a state of the switch section recorded by programming is read out by applying the third potential to the third control terminal and temporarily placing the first and second switching elements into on states, and then stopping the preservation signal to thereby release short-circuited states of the first and second ferroelectric capacitors and place the third and fourth switching elements into on states, and applying a potential, which is higher than the third potential, at the third control terminal to thereby operate the signal preservation section.
The first and second short sections are each formed by one of an enhancement-type field effect transistor and a depletion-type field effect transistor. Further, the first through fourth switching elements are each formed by one of an enhancement-type field effect transistor and a depletion-type field effect transistor. The circuit may also include a first capacitor connected at one side thereof to the first control terminal and the third switching element and connected at another side thereof to earth; and a second capacitor connected at one side thereof to the second control terminal and the fourth switching element and connected at another side thereof to earth.
Thus, the first and second short sections are connected in parallel with the first and second ferroelectric capacitors. When the first and second short sections are made to form short-circuits by application of the preservation signal, voltages between the input/output terminals of the first and second ferroelectric capacitors are close to 0 V. Thus, the risk of imprinting is removed, and polarization states can be preserved for long periods of time.